Elsevier

Science Bulletin

Volume 65, Issue 7, 15 April 2020, Pages 525-530
Science Bulletin

Article
Fast jet proper motion discovered in a blazar at z=4.72

https://doi.org/10.1016/j.scib.2020.01.008Get rights and content

Abstract

High-resolution observations of high-redshift (z>4) radio quasars offer a unique insight into jet kinematics at early cosmological epochs, as well as constraints on cosmological model parameters. Due to the general weakness of extremely distant objects and the apparently slow structural changes caused by cosmological time dilation, only a couple of high-redshift quasars (HRQs) have been studied with parsec-scale resolutions, and with limited number of observing epochs. Here we report on very long baseline interferometry (VLBI) observations of a high-redshift blazar J1430 + 4204 (z=4.72) in the 8 GHz frequency band at five different epochs spanning 22 years. The source shows a compact core–jet structure with two jet components being identified within 3 milli-arcsecond (mas) scale. The long time span and multiple-epoch data allow for the kinematic studies of the jet components. That results in a jet proper motion of μ(J1) = 0.017 ± 0.002 mas a-1 and μ(J2) = 0.156 ± 0.015 mas a-1, respectively. For the fastest-moving outer jet component J2, the corresponding apparent transverse speed is (19.5±1.9)c. The inferred bulk jet Lorentz factor Γ=14.6±3.8 and viewing angle θ=2.2°±1.6° indicate highly relativistic beaming. The Lorentz factor and apparent proper motion are the highest measured to date among the z>4 jetted radio sources, while the jet kinematics is still consistent with the cosmological interpretation of quasar redshifts.

Graphical abstract

Jet component positions with respect to the core as a function of time and the fitted proper motions. The left and right panels show the projected motions along and perpendicular to the jet direction, respectively.

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Introduction

Active galactic nuclei (AGNs) are among the most powerful and energetic objects in the Universe. It is believed that every AGN harbours a supermassive black hole (SMBH) in its central region. Because of their enormous energy production and persistent high luminosity, AGNs are excellent laboratories for studying black hole accretion and galaxy evolution across cosmic times. High-redshift quasars (HRQs) that represent powerful AGNs in the early Universe constitute a unique sample for evolutionary studies of AGNs as well as of the cosmic environment [1], [2]. The most distant AGN known to date is the quasar J1342 + 0928 at z=7.54, corresponding to only 5% of the current age of the Universe [3]. The number of quasars known at z>5.6 already exceeds 100 (e.g., Refs. [4], [5]). The discovery of extremely HRQs provided strong constraints on the formation history of the first generation of SMBHs [3], [6].

Among the HRQs, the radio-loud subsample is worth for exploring their relativistic jets and radio morphologies. Jets from radio-loud HRQs are useful probes of the intergalactic and interstellar environment in the very early stages (e.g., the epoch of reionization). Imaging of HRQ jets requires milli-arcsecond (mas) resolution or even higher. This can be achieved by using very long baseline interferometry (VLBI), which is the highest-resolution imaging technique. The facts that only 10% of the optically detected quasars are radio-loud, and that radio-loud quasars become too weak to be detected by radio telescopes at very large cosmological distances make high-redshift radio-loud quasars much more rare. The number of radio-detected AGNs with available spectroscopic redshift at z>4 is only about 170 [7], and the most distant radio-loud quasar to date is J1429 + 5447 at z=6.21 [8], [9].

J1430 + 4204 was discovered as a radio-loud quasar at z=4.72 [10]. Further X-ray and radio observations confirmed its blazar nature with broad-band variability and flat radio spectrum [11], [12], [13]. A distant (3.6) jet component was detected in the northwestern direction of the nucleus from Chandra X-ray and Very Large Array (VLA) radio observations, making J1430 + 4204 the most distant quasar being detected with kiloparsec (kpc) scale radio/X-ray jet known so far [14], [15]. The mas-scale radio structure of J1430 + 4204 has been studied with VLBI since 1996 [16]. With further more sensitive and higher-resolution VLBI observations, a compact core–jet structure was revealed, with a weak mas-scale jet ejecting towards the west-southwest seen at 2.3, 8.4 and 5 GHz frequencies (e.g., Refs. [15], [17], [18]). Veres et al. [19] conducted a two-epoch 15-GHz VLBI study of J1430 + 4204 before and after a major radio flare from 2005 to 2006. They did not find any evidence of newly ejected jet components. The jet emission in their 15-GHz VLBI images appears diffuse, hampering a direct measurement of jet proper motion.

In this paper, we present a multi-epoch VLBI analysis of J1430 + 4204. Thanks to the long time span and multiple epochs, we are able to study the jet kinematics of J1430 + 4204. The five-epoch data were obtained in the 8-GHz frequency band, where the jet components are bright and compact enough for model-fitting. In Section 2, we provide the basic information about our VLBI data and describe their analysis. Section 3 presents the high-resolution images, the results of brightness distribution modeling, and the calculation of jet parameters. In Section 4, we discuss the properties of J1430 + 4204 and put them into context with other extremely distant quasars studied with VLBI.

Throughout this paper, a flat ΛCDM cosmological model was adopted, with the parameters of H0=70 km s-1 Mpc-1,Ωm=0.27, and ΩΛ=0.73. At the redshift of J1430 + 4204, 1 mas angular size corresponds to 6.68 pc projected linear size and 1 mas a-1 proper motion to 124.7c apparent transverse speed (c denotes the speed of light) [20].

Section snippets

Methods

Data used for the jet kinematic study were obtained from VLBI observations in a total of five epochs from June 8, 1996 to May 1, 2018 (see the Supplementary materials). All these observations were made in geodetic/astrometric mode, in which a target source is observed during a few scans, usually for 5 min in each scan. The details of the observation logs are presented in Table B1 (online). Except for the fourth epoch observed only at 8.4 GHz, the other observations were carried out at dual

Parsec-scale radio jet

Because of the limited sensitivity of geodetic-style “snapshot” VLBI experiments with short on-source integration times and poor (u,v) coverage (Table B1 online), not all epochs are sufficient to produce a reasonably detailed image of the core–jet structure. At 2.3 GHz band, we show only the best-quality image made from the epoch 2001 data (Fig. 1a) to indicate that the jet emission continues expanding out to an extent 20 mas (or a projected distance of 130 pc) from the core (the brightest

Discussion and conclusion

According to the unified scheme of radio-loud (jetted) AGNs, the appearance and luminosity of the objects depend on their jet orientation with respect to the viewing direction [32]. Blazars, whose relativistic jets point nearly towards us, are intriguing sources to probe the early Universe. Their jet emission is strongly enhanced due to the Doppler beaming effect which makes them more easily detectable compared to the coetaneous quasars with unbeamed jets, especially at high redshifts (e.g.,

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgments

This work was supported by the National Key R&D Programme of China (2018YFA0404603), the Chinese Academy of Sciences (114231KYSB20170003), and the Hungarian National Research, Development and Innovation Office (2018-2.1.14-TÉT-CN-2018-00001). The authors acknowledge the use of Astrogeo Center database of brightness distributions, correlated flux densities, and images of compact radio sources produced with VLBI. YZ thanks Shu Fengchun, Alexey Melnikov, Jamie McCallum, and Bo Xia for providing

Yingkang Zhang is a Ph.D. student at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. He got his B.S. degree at Hebei University of Technology, China. He began his study on astrophysics in Shanghai Astronomical Observatory in 2014. His research fields are high-redshift AGN and very long baseline interferometry (VLBI).

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    Tao An is a professor at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. He is Member of Square Kilometre Array (SKA) Regional Centre Streering Committee and International Astronomical Society (IAU) Commission B4, co-chair of SKA VLBI science working group. He is leading the China SKA Regional Centre prototype construction. His research fields are astrophysics, radio astronomy, and very long baseline interferometry (VLBI).

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